As a first step of a program aimed to the detection of dark matter (or radial variations of M/L) in early-type galaxies, we report deep spectroscopic observations of the bulge-dominated edge-on S0 galaxy NGC 3115, made at ESO, La Silla, using EFOSC at the 3.6m telescope and EMMI at NTT. Such observations allow measurements of the rotational velocity out to 1.8 a_e (effective radii) from the galaxy center, where the surface brightness is ��_B ~ 24 mag arcsec**-2. The rotation curve quickly reaches an asymptotic value, v_f ~ 260 km/s, with only marginal indication of systematic decline within the range of our observations. The line-of-sight velocity dispersion has also been measured; it decreases steeply from a rather high central value and flattens out ( ~ 100 km/s) within our observing range (a ~< 1.3 a_e). Models built on these data and simple dynamical arguments show that the M/L of NGC 3115 must thus be increasing from M/L = 6 (in solar units) in the inner regions (~ 1 a_e) to at least M/L >= 10 in the outermost regions (~ 2 a_e).

Latex + L-AA Vers. 3.0, figures available from the authors